Playing a detective

March 30, 2013

For an entomologist few pleasures in life are greater than arriving in a new geographic area and being stumped by unfamiliar and mysterious insects, often ones that he/she had never suspected existed. I had a moment like this last night, when I ran across a strange, metallic blue insect, about 25 mm long, walking on the sand in Chitengo. My first thought was that it must be a blattodean, some of which are wingless and have a dark, metallic sheen. But when I picked it up I realized that it was a beetle larva. I knew it because the insect lacked well-developed tarsi or any traces of wings or wing pads, and its antennae consisted of only two articles (although one was exceptionally long). But what family? Ground beetles (Carabidae) often have strange, active larvae that hunt insects in sandy areas, and this was my guess.

Photo: Mysterious larvae in a tree cavity in Gorongosa.

Then I happened to look up at the trunk of a nearby tree and saw several dozens of these larvae huddled in a small cavity about 2 m above the ground; the one I found must have fallen off the tree and was trying to find its way back home. I concluded that it couldn’t be a carabid as their larvae are ground-dwelling predators and thus unlikely to (a) live high in the trees and (b) form large aggregations.

A few meters away another tree had a second colony of these insects, but this one was a little older. Although it still had a few larvae moving around, most had already metamorphosed into pupae, which were hanging in grape-like clusters, eerily reminiscent of a scene from the movie “Alien”. There were about 50 of them hanging together, but the tree cavity was very deep and narrow, and I couldn’t get a good photo of the group. I had never seen beetle pupae hanging in a similar formation. The mystery deepened.

Photo: A cluster of pupae in a tree cavity.

I shone a light into the cavity and squeezed my hand in to scoop one of the pupae. At that moment I noticed two adult beetles, which seemed to be guarding the cluster. They were clearly a male and a female since one individual was slightly larger and had distinctly thickened front legs with a pair of large spines; I assumed that it was the male. Both beetles ran away when I put my hand in, but quickly returned and assumed the same position near the clutch of pupae.

And they were darkling beetles, or Tenebrionidae! One of the first things you learn in an entomology class is that tenebrionids have elongate, vermiform larvae that burrow in the ground, and for this reason it never crossed my mind that the blue, free-running larvae in the tree might belong to this family. A cursory search of coleopterological literature revealed that my beetles may be members of a large, nearly cosmopolitan genus Strongylium, of which some species are arboreal. If any any entomologist reading this has another idea, I would love to know it – being stumped by an unknown insect is a pleasure, but never learning its identity is torture.

Photo: Three developmental stages of a mystery beetle, found together in the same tree “colony”, tentatively identified as Strongylium sp. (Tenebrionidae).